Optical heat flux gauge

ABSTRACT

A heat flux gauge comprising first and second thermographic phosphor layers separated by a layer of a thermal insulator wherein each thermographic layer comprises a plurality of respective thermographic phosphors. The gauge may be mounted on a surface with the first thermographic phosphor in contact with the surface. A light source is directed at the gauge, causing the phosphors to luminesce. The luminescence produced by the phosphors is collected and its spectra analyzed in order to determine the heat flux on the surface. First and second phosphor layers must be different materials to assure that the spectral lines collected will be distinguishable.

BACKGROUND OF THE INVENTION

This invention relates generally to the measurement of heat flux, thatis the measurement of the amount of heat transferred across a surfaceper unit area per unit time, and, more specifically, to the measurementof heat flux utilizing the optical properties of thermographicphosphors. The invention is a result of a contract with the Departmentof Energy (Contract No. W-7405-ENG-36).

The measurement of heat flux is important in many experimentalsituations, such as those where heat transfer must be limited andtherefore monitored. For example, accurate measurement of heat-transferrates is considered critical to the design improvements envisioned forhigh-pressure turbine engines. Improved understanding of the effectsthat contribute to heat load can lead to increased efficiency. Ofparticular interest is the heat transferred from the free-stream gas toan engine component surface. Examples include turbine blades and vanes.

Previous heat flux gauges have principally involved some form ofresistance thermometer temperature sensor applied on both sides of aninsulating medium as conducting surfaces. These conducting surfaces canalso be made from pairs of materials in a thermocouple configuration.Leads connected to these surfaces would carry an electrical currentwhich is proportional to the surface temperature detected by the sensorto an external instrument which would measure the temperatures of thesurfaces. A typical gauge is made by depositing thin layers of anelectrically and thermally conductive material onto both sides of a thinsheet of insulating material such as Mylar® or Kapton®.

Heat flux, Q, incident on an ideal gauge made in this way is given bythe following equation:

    Q=k(delta T)/L; 10

where k is the thermal conductivity of the insulator, L is the thicknessof the insulator, and delta T is the temperature difference between thetwo conductive surfaces. This equation assumes that the conductivesurfaces are infinitely conducting and infinitely thin.

Modern embodiments of this configuration are disclosed in U.S. Pat. Nos.4,779,994, 4,722,609, and 4,577,976.

U.S. Pat. No. 4,779,994 to Diller, et al. discloses a fairlyconventional heat flux gauge which utilizes thin film layers applied toeach side of a planar thermal resistance element, with its "cold"junctions applied to one surface and its "hot" junctions applied to theother. The use of thin films allows the deposition of a large number ofjunctions onto a small surface area which can be interconnected inseries. Of course, these junctions are of the electrical resistancetype, and require electrical connections.

U.S. Pat. No. 4,722,609 to Epstein et al. discloses a double sided,high-frequency response heat flux gauge consisting of a metal filmapproximately 1500 Angstroms thick applied to both sides of a thin (25micrometer) polyimide sheet. At low frequencies, the temperaturedifference across the polyimide is a direct measure of the heat flux. Athigher frequencies, a quasi-one-dimensional assumption is used to inferthe heat flux. Numerous such gauges are arranged in a serpentine patternand applied to the surface of a turbine blade.

Yet another thin film heat flux gauge is disclosed in U.S. Pat. No.4,577,976 to Hayashi et al. wherein a pair of metallic thin films areattached to opposite surfaces of a heat resistive thin film. The heatflux through the heat resistive film is determined by measuring thetemperature gradient therein while using the metallic thin films asresistance thermometer elements.

The pervading problem plaguing the above heat flux gauges, as well asall prior art heat flux gauges, is that they are electrically based.Thus, they all require connecting wires of some type in order tooperate. This complicates their use, and severely limits theirapplication to rotating components, as wire connections would have to bethrough slip rings. This severely complicates such an application, andgreatly detracts from its reliability.

Connecting leads or wires of the prior art also limit the spatialresolution when multiple heat flux gauges are needed to measure thespatial distribution of heat flux. The degree of complication, becauseof the inherent geometry of such electrically based gauges, effectivelyprecludes their use in measuring the spatial distribution of heat fluxwith acceptable resolution and areal coverage. Wiring dozens of gaugesis complicated and interferes with the natural heat transfer to or fromthe surface under test. Connecting wires also present problems when suchgauges are used in hostile environments.

The current invention solves the problems of the prior art by providinga leadless heat flux gauge that uses light instead of electrical meansas its interrogating medium. The sensing elements of the gauges arethermographic phosphors, whose emission lines in the luminescencespectrum are temperature dependent. This allows accurate temperaturedetermination when the phosphors are interrogated by ultraviolet light,and the spectral lines of the emitted light is analyzed. It also allowsfor a heat flux gauge requiring no electrical connections between thegauge and the associated evaluation and display equipment.

It is therefore an object of the present invention to provide apparatusfor the accurate measurement of heat flux.

It is another object of the present invention to provide a heat fluxgauge that does not require electrical connections.

It is still another object of the present invention to provide a heatflux gauge that will operate in a hostile environment.

It is still another object of the present invention to provide a heatflux gauge that is interrogated with light.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description that follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the apparatus of this invention may comprise a multilayer heatflux gauge for measuring heat flux on a surface comprising first andsecond thermographic phosphor films separated by an insulating mediumand bonded to the surface. Light means focused on the first and secondphosphor films produce emitted light indicative of the temperature ofthe first and second phosphor films. Interpreting means receive theemitted light and produce an output indicative of the heat flux on thesurface.

In another aspect of the present invention, and in accordance andpurposes, an optically interrogated gauge for measuring heat fluxincident on a plurality of points on a surface may comprise a pluralityof first thermographic phosphors in direct contact with the surface witha plurality of second thermographic phosphors overlying and spaced apartfrom the plurality of first thermographic phosphors. Thermal insulatingmeans are located between the plurality of first thermographic phosphorsand the plurality of second thermographic phosphors for thermallyisolating the plurality of first thermographic phosphors from theplurality of second thermographic phosphors. Light means are incident onthe plurality of first thermographic phosphors, for producing firstluminescence from each of the first plurality of thermographic phosphorsand second luminescence from each of the plurality of secondthermographic phosphors, wherein the first luminescence and the secondluminescence are indicative of the temperatures of the first and secondthermographic phosphors at a plurality of points on the surface.

In a still further aspect of the present invention, an opticallyinterrogated gauge for measuring heat flux incident on a plurality ofpoints on a surface may comprise a plurality of first thermographicphosphors in direct contact with the surface and a plurality of secondthermographic phosphors spaced above, but juxtaposed in relation to theplurality of first thermographic phosphors. Thermal insulating means arelocated between the plurality of first thermographic phosphors and theplurality of second thermographic phosphors for thermally isolating theplurality of first thermographic phosphors from the plurality of secondthermographic phosphors. Light means incident on the plurality of firstthermographic phosphors and on the plurality of second thermographicphosphors, produce first luminescence from each of the plurality offirst thermographic phosphors and second luminescence from each of theplurality of second thermographic phosphors, wherein the firstluminescence and the second luminescence are indicative of thetemperatures of the first and second thermographic phosphors at aplurality of points on said surface.

In a still further aspect of the present invention, and in accordancewith its objects and purposes, a method of determining heat flux on asurface may comprise focusing a light source on first and secondthermographic phosphor films separated by an insulating medium andbonded to the surface; interpreting the light emitted from the first andsecond phosphor films to determine the temperature of the phosphor filmsand the heat flux on the surface; producing an output indicative of theheat flux on the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is an enlarged schematic drawing of a phosphor heat flux gaugeaccording to the present invention.

FIG. 2 is a schematic representation of a possible practicalconfiguration for application of the present invention.

FIG. 3 shows the luminescence spectra for a Gd₂ O₂ S:Tb phosphor layerand a La₂ O₂ S:Eu phosphor layer operating at 51° F.

FIG. 4 is another luminescence spectra of Gd₂ O₂ S:Tb and La₂ O₂ S:Euphosphor layers operating at 60° F., 75° F. and 91° F.

FIG. 5 contains plots of luminescence intensity for a Gd₂ O₂ S:Tbphosphor layer at the 415 nm and 490 nm lines normalized to 70° F.

FIG. 6 contains plots of luminescence intensity for a La₂ O₂ S:Euphosphor layer at the 511 nm and 614 nm lines normalized to 70° F.

FIG. 7 is a plot of intensity ratio versus temperature (°F.) for the 415nm/490 nm peak ratios of a Gd₂ O₂ S:Tb phosphor layer.

FIG. 8 is a plot of intensity ratio versus temperature (°F.) for the 511nm/614 nm peak ratios of a La₂ O₂ S:Eu phosphor layer.

FIG. 9 shows a second embodiment of the invention which includes aplurality of sensors.

DETAILED DESCRIPTION

The present invention allows accurate determination of heat flux on asurface without need for electrical connections through the use ofmultiple layers of thermographic inorganic phosphors applied to bothsides of a thermal insulator. The phosphors are interrogated with lightto determine the heat flux. The invention can be best understood byreferring to the drawings. In FIG. 1, a heat flux sensor according tothe present is illustrated as an enlarged schematic wherein surfaceunder test 10 has attached to it the assembly comprising firstthermographic phosphor layer 12, insulative layer 14, and secondthermographic phosphor layer 16, which together comprise sensor 5. Alsoshown is light source 18, and emissions 19, 20 from first thermographicphosphor layer 12 and second thermographic phosphor layer 16respectively. Heat flux, Q, incident on surface 10, is illustrated byarrow 17.

To insure that incident heat flux 17 can be accurately determined, it isnecessary that first thermographic phosphor layer 12 comprise athermographic phosphor different than the thermographic phosphor whichcomprises second thermographic phosphor layer 16. This is so that thedifferent materials will exhibit different spectral lines for the sameor different temperatures of layers 12, 16.

The choice of the materials for first thermographic phosphor layer 12and second thermographic phosphor layer 16 initially involves choosingthermographic phosphors which have high rates of change in emissionspectra over the temperature range of interest for surface 10.Presently, thermographic phosphors are available over the range of 0° K.to 2600° K.

Insulative layer 14 may be any thermal insulator material that issuitable for a particular application. That is, it must be transparentat all of the involved wavelengths, it should contribute minimalbackground luminescence, and, depending on the intended application, itmay need to have a high melting point. In one embodiment, insulativelayer 14 is polymethylpentene (PMP). PMP is highly transparent at one ofthe desirable excitation wavelengths for phosphor (254 nm), contributesa comparably minimal amount of background luminescence between 400 nmand 620 nm, and has a reported melting point of 455° F. One problem withmany of the currently available thermal insulator materials is that theywill melt at very high temperatures. It is for this reason thatcurrently glass would be the material of preference for very hightemperature applications.

Light source 18, in one embodiment, will be an ultraviolet light sourcefocused on sensor 5. With an ultraviolet light source it will benecessary to choose one having high efficiency, and accuracy. It shouldoperate at a wavelength which will most efficiently excite the desiredspectral lines of the phosphors. A wavelength that is acceptable formany applications, and that is readily available from commerciallyavailable UV lamps, such as a mercury arc lamp is 254 nm. Of course, anylight source that can produce a wavelength slightly shorter than theshortest wavelength emission line of the phosphor can be used to produceemissions 19, 20.

Emissions 19, 20 contain sufficient information through interpretationof their spectral lines to determine the temperatures of firstthermographic phosphor layer 12 and second thermographic phosphor layer16. With the temperature information, the heat flux, Q, can becalculated using equation 10.

One embodiment of a means for collecting and interpreting emissions 19,20 is illustrated in FIG. 2. Referring to FIG. 2, light from lamp 11 isfocused by lens 13 onto the top surface of sensor 5. As a result,emissions 19, 20, being phosphor luminescence, are emitted from sensor5, are collected by lens 22 and inserted into optical fiber 24 fortransmission into spectrometer 26 through a lens coupler (not shown).

Sensor 5, if an integral unit, can be attached to surface 10 using anyhigh thermal conductivity epoxy. Alternatively, using an air brush (notshown), first thermographic phosphor layer 12 could be deposited as athin layer directly onto surface 29, with a thin layer of insulator 14deposited on first thermographic phosphor layer 12. Second thermographicphosphor layer 16 would then be deposited as a thin layer on top ofinsulator 14. If desired for protection of sensor 5, a thin layer of aplastic material having high thermal conductivity could be depositedover second thermographic phosphor layer 16. It is important that thephosphor layers 12, 16 be thin enough to be thermally insignificant andto permit a substantial portion of light from lamp 11 to pass throughlayer 16 to layer 12. If sensor 5 is to be an integral unit, it is alsoconvenient to apply phosphor layers 12, 16 to insulator 14 is with anair brush. With the air brush, the layers may be applied using acetonecontaining a small quantity of dissolved adhesive as a carrier.

In spectrometer 26 the luminescence signal is dispersed and collected ondiode array 28, which, for example, may be an EG&G Reticon® diode array.The data from diode array 28 is recorded and processed by opticalmultichannel analyzer 30, which may be an EG&G model 1460 opticalmultichannel analyzer. Optical multichannel analyzer (OMCA) 30, havinginternal computer 30a, is a conventional multichannel analyzer, andconverts light incoming on optical fiber 24 into electrical signals.These electrical signals are then analyzed by internal computer 30a todetermine the heat flux according to equation 10, using the temperaturedifferences between phosphor layer 12 and phosphor layer 16, and thepredetermined thickness and thermal conductivity of insulative layer 14.

Typical results obtained from analyzer 30 are illustrated in FIG. 3. Theemission lines at 415 nm and 490 nm are from a Gd₂ O₂ S:Tb phosphorlayer 12 (FIG. 1). The emission lines at 511 nm and 614 nm are from aLa₂ O₂ S:Eu phosphor layer 16 (FIG. 1). These results are with the gaugeoperating at 51° F. A spectrum such as this is used to calibrate thegauge by determining luminescence intensity for each layer 12, 16 atseveral known temperatures.

To calibrate the gauge, it is necessary to maintain the same temperatureat both phosphor layers 12, 16. With this accomplished, the curves shownin FIG. 4 were obtained for the wavelength range of 400 nm to 520 nm.Curve A, shown as a light solid line, is for a temperature of 60° F.;curve B, shown as a dashed line, is for a temperature of 75° F.; andcurve C, shown as a heavy solid line, is for a temperature of 91° F.

The data thus obtained were normalized to the integrated intensity at70° F. Intensity values were calculated by integrating the luminescencespectrum over a selected bandwidth. FIGS. 5 and 6 present intensityratios versus temperature for the key emission lines in La₂ O₂ S:Eu andGd₂ O₂ S:Tb respectively. The continuum luminescence dilutes thetemperature sensitivity and the accuracy of these bands. By integratingabove a line connecting the extremes of the chosen bandwidth, thecontinuum is removed and the temperature, sensitivity and accuracy areincreased. For La₂ O₂ S:Eu (FIG. 6), the 511 nm line decreases rapidlybetween 70° F. and 95° F., whereas the reference line at 614 nmincreases slightly. With Gd₂ O₂ S:Tb (FIG. 5), the 415 nm line is themore sensitive in the temperature range of interest.

The ratios of the integrated intensities obtained from FIGS. 5 and 6(415/490 and 511/614 respectively) are plotted against temperature toproduce calibration curves for the gauge. The curve for Gd₂ O₂ S:Tb(415/490) is illustrated in FIG. 7, and the curve for La₂ O₂ S:Eu(511/614) is illustrated in FIG. 8. In the temperature range between 70°F. and 95° F., La₂ O₂ S:Eu is a more sensitive thermometer than Gd₂ O₂S:Tb. In this range, the La₂ O₂ S:Eu (511/614) peak ratio changes at anaverage rate of 3.3% per °F., whereas the Gd₂ O₂ S:Tb (415/490) ratiochanges at a rate of 0.83% per °F. After the gauge is calibrated, asteady-state heat flux can be measured at a single point on thestationary surface under investigation.

In another embodiment, many sensors 5 could be deposited on surface 10to determine spatial distribution of heat flux. Such an arrangement isshown in FIG. 9. In this situation, the entire surface could be floodedwith light. Or, the light from lamp 11 (FIG. 2) can be scanned acrosssensors 5 and the emissions from individual sensors 5 can be gathered.By this method, flux rates for discrete areas of surface 40 can bemonitored.

In yet another embodiment (not illustrated), sensor 5 can be constructedso that light from lamp 11 (FIG. 2) does not have to pass throughphosphor layer 16 in order to reach phosphor layer 12, allowing for moreaccurate temperature measurements. In this embodiment, using thenumbering of components in FIG. 1, a checkerboard of individual phosphorlayers 12 are applied to surface 10. A layer of insulator 14 isdeposited over layers 12, and a checkerboard of phosphor layers 16,juxtaposed in relation to layers 12, is deposited over insulator 14.This allows layers 12 to receive the full light available from lamp 11,except for the usual minor losses in insulator 14, while a continuousluminescence is produced by layers 12 and layers 16 to lens 22.

The foregoing description of embodiments of the present invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed, and obviously many modifications and variations are possiblein light of the above teachings. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical applications to thereby enable others skilled in the artto best utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

What is claimed is:
 1. An optically interrogated gauge for measuringheat flux incident on a plurality of points on a surface comprising:aplurality of first thermographic phosphors in direct contact with saidsurface; a plurality of second thermographic phosphors overlying andspaced apart from said plurality of first thermographic phosphors;thermal insulating means having a predetermined thickness and thermalconductivity between said plurality of first thermographic phosphors andsaid plurality of second thermographic phosphors for thermally isolatingsaid plurality of first thermographic phosphors from said plurality ofsecond thermographic phosphors; light means incident on said pluralityof first and second thermographic phosphors, for producing firstluminescence from each of said first plurality of thermographicphosphors and second luminescence from each of said plurality of secondthermographic phosphors, collecting means for collecting said first andsecond luminescences, wherein said first luminescence and said secondluminescence are indicative of the temperatures of said first and secondthermographic phosphors at a plurality of points on said surface; andcomputing means connected to said collecting means for determining heatflux on said plurality of points on said surface using numericaldifferences between said temperatures of said pluralities of first andsecond thermographic phosphors, and said thickness and thermalconductivity of said thermal insulating means.
 2. The heat flux gauge asdescribed in claim 1, wherein said thermal insulating means istransparent to light having wavelengths in the area of 254 nm.
 3. Theheat flux gauge as described in claim 1, wherein said thermal insulatingmeans comprises polymethylpentene (PMP).
 4. The heat flux gauge asdescribed in claim 1, wherein said plurality of first thermographicphosphors comprises Gd₂ O₂ S:Tb.
 5. The heat flux gauge as described inclaim 1, wherein said plurality of second thermographic phosphorscomprises La₂ O₂ S:Eu.
 6. The heat flux gauge as described in claim 1,wherein said light means comprises a mercury lamp.
 7. The heat fluxgauge as described in claim 1, wherein said light means is scannedacross said plurality of first and second thermographic phosphors.